The subject matter disclosed herein relates to an air cycle machine and, more particularly, to a cooling air supply control system for an air cycle machine.
Conventional aircraft environmental control systems (ECSs) incorporate an air cycle machine, also referred to as an air cycle cooling machine, for use in cooling and dehumidifying air for an aircraft cabin. Such air cycle machines may include two or more wheels disposed at axially spaced intervals along a common shaft. The wheels are part of, for example, a compressor rotor, a turbine rotor, a fan rotor, an additional turbine rotor, or an additional compressor rotor. In some cases the turbine or turbines drive both the compressor and the fan.
On aircraft powered by turbine engines, the air to be conditioned in the air cycle machine is typically compressed air bled from one or more of compressor stages of the turbine engine. In conventional systems, this bleed air passes through the air cycle machine compressor where it is further compressed. The compressed air is passed through a heat exchanger to cool the compressed air sufficiently to remove moisture and dehumidify the air. The dehumidified compressed air is expanded in the turbine of the air cycle machine to both extract energy from the compressed air so as to drive the shaft and also to cool the expanded turbine exhaust air before it is supplied to the aircraft cabin as conditioned cooling air.
A flow path of an air cycle machine can also include a heat exchanger cooling flow that draws air through the heat exchanger, past a fan rotor, and dumps the flow into an overboard duct. The fan rotor can be used to establish the flow when insufficient ram air is available to draw air through the heat exchanger.
Bearings are used and employed within air cycle machines. As the air cycle machine operates, the bearings will heat up. The heat can lead to damage to the bearings or to other components of the air cycle machine. Thus, bearing cooling air is fed into the air cycle machine for the purpose of maintaining operational temperatures for the bearings i.e., relatively cool temperatures.
Hydrodynamic fluid film journal bearings, also called journal air bearings or foil bearings, can be used to provide support to rotatable components such as shafts. A typical journal bearing may include a journal sleeve, a bump foil, an intermediate foil, and a top foil. During operation, rotation of the rotatable component causes a working fluid to form a cushion (often referred to as an “air bearing”) that supports the rotatable component with little or no direct contact between the rotatable component and the foils of the bearing. Journal bearings provide fluid cushions for radial loads.
Similarly, hydrodynamic fluid film thrust bearings generate a lubricating non-linear air film between a portion of a rotating shaft or other rotatable component and the bearing. One typical bearing arrangement utilizes welded subassemblies. A top subassembly includes an annular main plate having multiple arcuate, corrugated foils welded to the main plate. A corresponding number of arcuate top foils are supported by bump foils. A bottom subassembly includes another annular main plate having multiple arcuate bump foils welded to the main plate. Thus, during operation, rotation of the rotatable component or shaft causes a working fluid to form in and around the corrugated foils to provide an air bearing. The bump foils provide a desired spring rate to cushion the rotatable component as the shaft moves axially. Thus, thrust bearings provide fluid cushions for axial loads.
During operation, the bearings may be rotated at high speeds which result in heat generation. The heat can lead to failure of the bearings by compromising the structural integrity of the components of the bearings. To reduce the risk of failure of the bearings, cooling air is conveyed and passed over bearing surfaces to remove the heat from the bearing.
Traditionally, the bearing cooling flow is supplied from a single high pressure, cool temperature source. For example, the bearing cooling flow is traditionally sourced from the turbine inlet of the air conditioning system, with the source of the air to the air conditioning system a compressor stage of the engine of the aircraft. Check valves may be used to close the cooling air inlet to close the bearing cooling circuit in order to reduce leakage and impact system efficiency when the air cycle machine is not running, effectively shutting off the cooling air supply when not in use.